Positron emission tomography is a diagnostic imaging modality that is used to non-invasively measure the bio-distribution of a radioactive tracer. In positron emission tomography, a positron emitting bare radioactive isotope or an isotope that has been attached to a chemical molecule, is injected into a patient or animal. A positron is emitted by the radioactive isotope and annihilates with an electron producing two photons in opposite directions. Each of the photons has approximately 511 keV of energy, corresponding to the mass of the positron and electron. These two annihilation photons escape the patient and interact in a scanner that is positioned around the patient.
A scanner is made of arrays of high energy photon detectors that convert interactions in the detector into electrical signals that are processed on a computer. An example of a high energy photon detector is a scintillation crystal that is connected to an optical photodetector such as a photomultiplier tube or solid state photomultiplier. The photon is classified as high energy because the photon has an energy that 511 keV, or kila electron volt, which is much larger than optical photons that have energies in the 2-5 eV range. The annihilation photon can interact in the high-Z dense scintillation crystal, which in turn emits blue photons that bounce inside of the scintillation crystal. The blue optical photons propagate inside the crystal and are absorbed by a photodetector converting the light into an electrical signal. The electrical signal is then processed by analog and digital electronic circuits and is recorded as an event. The data acquisition electronics process the signal and records the time, location of the crystal or crystals that absorbed the high energy photon and any secondary interaction processes, and the energy of sum energy of the incoming high energy annihilation photon to storage. In positron emission tomography, the two photons are paired by their timestamps to produce a line-of-response (LOR) of the interaction. These LORs are processed by image reconstruction algorithms to produce 3-D images of the distribution of the radiotracer. High energy photon detector elements are placed around the object to be imaged covering a certain solid angle or angular coverage. The solid angle, or angular coverage around the object to be imaged, plus the efficiency of stopping and detecting the annihilations photons determines the sensitivity of the scanner. A scanner with a higher sensitivity will potentially have a better image quality or a shorter scan time than a scanner with a lower sensitivity. The cost of a scanner is directly related to the number of detection elements in the system. The scanning geometry is designed to optimize the sensitivity as a function of cost, size, and disposition of the object being imaged. The high energy photon detectors have depth-of-interaction capability to remove the blurring that results from photons that penetrate into the crystal. Better the depth resolving capability of the depth-of-interaction detector will result in a more uniform spatial resolution.
A time-of-flight scanner is one where the arrival time of the photons are recorded to such an extent that the annihilation location can be estimated. Because annihilation photons travel at the speed of light, the annihilation location can be estimated by the following equation: delta_x=delta_t/2*c), where delta_x is location of the annihilation measured from the center of the line, delta_t is the difference in time measured by the detectors, and c is the speed of light. Time-of-flight information can significantly improve limited angle PET by providing information that was lost.